Aluminium metal is generally extracted from alumina (Al2O3) electrolytically by a method commonly referred to as the Hall Héroult process. This process is well known to practitioners in the aluminium industry, and needs no further discussion here.
Rather than directing attention to the process itself, the focus of this invention lies on the vessel or cell in which this electrolytic process is operated. The upper (anodic) portion of the cell is typically comprised of one or more current carrying (commonly carbonaceous) blocks intended to evenly distribute electrical current across a shallow (in the sense that it is of much greater dimension horizontally than vertically through its depth) liquid layer of molten cryolite, surmounting another layer of molten aluminium.
The lower (cathodic) portion of the cell physically contains the layers of molten cryolite and aluminium in a cavity formed of refractory materials, with the lower surface of that cavity again formed of electrically-conducting (commonly carbonaceous) material. That electrically-conducting material is commonly formed as a series of large blocks (cathode blocks), into which metallic current conductors (collector bars) are embedded to provide an assembly of paths for the electrical current to leave the cell.
It is common practice that a plurality of these cells are connected together as a series circuit by a system of busbars, enabling the electrical current to enter each cell in turn through its anodic portion, provide energy for the electrolytic process operated within the liquid cryolite and aluminium layers contained within the cathodic portion, and ultimately leave the cell through the collector bars.
As the electrical current traverses the cell, it naturally seeks the path of least resistance through the cell components, thereby directing the greatest concentration of the current towards the juncture at which the collector bars leave the cathode blocks. This uneven distribution of current has the deleterious effect of significantly increasing the consumption (generally by means of erosive processes) of the cathode blocks in the areas of highest current concentration.
Prior art demonstrates that the distribution of current across the cathode blocks can be significantly improved by use of a composite collector bar, consisting of an outer steel sheath enclosing a highly electrically-conductive (typically copper) core for part of its length. This improvement in current distribution is known to significantly improve the operational lives of the cathode blocks.
While these improved collector bars contribute towards lower cathode erosion and hence improve the operational lives of the cathode blocks, these benefits need to be weighed against the high fabrication costs related to the materials of construction and the complexity of assembly of the composite collector bar arrangements. Therefore, a need exists for a composite collector bar arrangement having the benefits of the complementary material arrangements, but which is relatively simpler to fabricate, thereby significantly reducing costs.